Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some insects, fishes, amphibians, molluscs, crustaceans, cnidarians, echinoderms and tunicates undergo metamorphosis, which is usually accompanied by a change of habitat or behavior.
Definition from Wikipedia: http://en.wikipedia.org/wiki/Metamorphosis